Electric power steering apparatus

ABSTRACT

An electric power steering apparatus comprises a ball nut rotationally driven by an electric motor through a gear train and having a female thread groove formed in the inner periphery thereof, a rack shaft disposed through the ball nut on the axis thereof and having a male thread groove so formed in its outer periphery as to face to the female thread groove, a ball screw mechanism having a number of circulation balls interposed between the female thread groove and the male thread groove, and a housing for holding the ball nut rotatably through the bearing. An elastic member is interposed between the housing and the ball nut in order to permit the ball nut to make a predetermined displacement.

This application claims the benefit of Japanese Patent Applications No.2001-194837, No. 2002-032774 and No. 2002-065602 which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ball screw rack assist type electricpower steering apparatus.

2. Related Background Art

Steering systems of automobiles widely use a so-called power steeringapparatus for assisting a steering operation by use of an external powersource. The power source for the power steering apparatus has hithertoinvolved the use of a hydraulic pump, and this hydraulic pump is drivenby the engine in many cases. This type of power steering apparatus has,however, a large power loss (on the order of several through tenhorsepower (HP) at a maximum load) of the engine because of driving thehydraulic pump at all times and is therefore hard to apply to mini-sizedmotor vehicles exhibiting small displacements. Even the automobileshaving comparatively large displacements, when installed, come to havesuch an inevitable demerit that their running fuel economy becomes lowenough not to be negligible.

Such being the case, an electric power steering (which will hereinafterbe abbreviated to EPS) involving the use of an electric motor as a powersource has attracted the attention over the recent years for obviatingthose problems. The EPS apparatus exhibits characteristics, whereinthere is no direct drive loss of the engine because of using a batteryloaded into the car as a power source for the electric motor, a decreasein the running fuel economy is restrained as the electric motor isstarted only when assisting the steering operation, and in addition theprecise electronic control can be extremely easily performed.

On the other hand, for a steering gear mechanism for a passenger car, arack-and-pinion steering gear is mainly employed at the present, sincethe rack-and-pinion gear has high rigidity performance and low inweight. Then, the EPS apparatuses for the rack-and-pinion steering gearinclude a column assist type in which the electric motor is arrangedsideways of the column in order to drive a steering shaft and a pinionitself, and in addition a ball screw rack assist type in which a rackshaft is driven by an electrically-driven ball screw mechanism. In theball screw rack assist type EPS apparatus (which will hereinafter besimply called the rack assist type EPS apparatus), the assist force doesnot act on a meshing surface between the pinion and the rack, andtherefore a contact surface pressure between these two members, which isto be a factor for abrasion and deformation as well, is relativelysmall.

According to the rack assist type EPS apparatus, a ball screw shaft malethread groove formed in a rack shaft engages with a female thread grooveformed in a ball nut through a multiplicity of circulation balls (steelballs), and the ball nut is rotated by the electric motor, therebymoving the rack shaft in the axial directions. A power transmissionmethod for transmitting the power from the electric motor to the ballnut may be a timing belt system disclosed in Japanese Utility ModelPublication No. 6-49489 etc, however, a general method is a gear systemdisclosed in Japanese Utility Model Publication Nos. 5-14939 (a firstprior art) and 2-46455 (a second prior art) etc. In case of theconventional rack assist type EPS apparatus, if low of an assemblyaccuracy of the respective parts constituting the ball screw mechanismand the rack and pinion mechanism, a misalignment occurs between theball nut and the rack and pinion pair. In this case, so-calleddistortions are formed between the ball nut and the rack shaft, with theresult that a smooth movement of the rack shaft is hindered. Thishindrance of the smooth movement might cause a loss of driving force, anabnormal abrasion and, besides, noises when operated. Further, if theworking accuracy and the assembly accuracy of the housings thataccommodate and hold the ball screw mechanism and the electric motor,are low, a backlash occurs between the gear pair connecting the ball nutand the electric motor together, and the noises are likewise emitted.

Such being the case, each of the rack assist type EPS apparatusesaccording to the first and second prior arts adopts an aligningmechanism for supporting the ball nut so as to be swayable about thehousing through a spherical bearing in order to obviate a misalignmentbetween the ball nut and the rack shaft. Large increases in the numberof manufacturing processes and in the costs for the product are,however, inevitable for providing the spherical bearing between thehousing and the ball nut. Moreover, the spherical bearing utilizes theslide on between the metal members, and hence, if a comparatively largeload acts, a predetermined aligning operation is not obtained due to arise in slide friction force, with the result that the misalignment isnot obviated in the great majority of cases. Further, if the ball nut issupported on the spherical bearing, a central position of the ball nutremained unchanged, so that the backlash between the gears is not, as amatter of course, canceled.

Generally the ball nut configuring the ball screw mechanism isstructured so that a rolling bearing (a double-row angular ball bearingetc) is fitted on and made integral with the ball nut body. The ball nutis fixed by a ring bolt or a C-shaped stopper ring brought into contactwith an outer ring of the rolling bearing within the housing.

In the conventional rack assist type EPS apparatus, the C-shaped stopperring comes off and falls out of an engagement groove formed in thehousing, or the ring bolt slackens and falls out due to a carelessoperation in the assembly and vibrations when driving. In this case,since the ball nut moves in an axial direction by dint of an externalforce, a proper axial force can not be given to the rack shaft. Thisresults in an extremely unagreeable feeling of the steering and in animpossibility of performing the precise steering.

According to the rack assist type EPS apparatus adopting the gear typepower transmission method, an idle gear is interposed between the gearon the electric motor and the gear on the ball nut in order to avoid anincrease in diameter of each of these two gears.

In the rack assist type EPS apparatus, the gear train for the powertransmission, the ball screw mechanism and the bearing for supportingthe ball nut, have hitherto been lubricated by respective lubricants(normally greases) for exclusive uses therefor. Hence, in this type ofEPS apparatus, three or four pieces of shaft seals are attached to sideportions etc of the gear portion of the ball nut, thereby preventing thelubricants from being mixed. If many shaft seals are thus used, a largefriction loss occurs when the ball nut rotates, there are caused a lossof driving torque and an unagreeable feeling of the steering. Further,the steering gear case and the ball nut need high-precision working ofseal seats and seal slide surfaces in order to accommodate and hold theshaft seals. This is a factor for increasing the costs for the productalong with the rise in the number of components.

SUMMARY OF THE INVENTION

It is an object of the first invention of the present application toprovide a rack assist type electric power steering apparatus capable ofimproving an operability and canceling a backlash etc in a gear train ofa driving system.

To accomplish the object given above, according to the first invention,an electric power steering apparatus comprises a ball nut rotationallydriven by an electric motor through a gear train and having a femalethread groove formed in the inner periphery thereof, a rack shaftdisposed through said ball nut on the axis thereof and having a malethread groove so formed in its outer periphery as to face to the femalethread groove of the ball nut, a ball screw mechanism having a number ofcirculation balls interposed between the female thread groove and themale thread groove, and a housing for holding the ball nut rotatablythrough the bearing, wherein elastic members are interposed between thehousing and the ball nut in order to permit the ball nut to make apredetermined displacement.

According to the first invention, even if a misalignment occurs betweenthe ball nut and the rack and pinion pair, the ball nut displaces with adeformation of the elastic member, thereby obviating the misalignment.Further, a backlash between the gears due to a defect in working of thehousing and so on, is also canceled by biasing the ball nut in adirection right-angled to the axis with the elastic member.

It is an object of the second invention of the present application toprovide an electric power steering apparatus capable of preventing afall-out of a fastening element for fixing the ball nut and thuspreventing a decline of a steerability.

To accomplish this object, according to the second invention, anelectric power steering apparatus comprises a ball nut rotationallydriven by an electric motor and having a female thread groove formed inthe inner periphery thereof, a rack shaft disposed through the ball nuton the axis thereof and having a male thread groove so formed in itsouter periphery as to face to the female thread groove, a number ofcirculation balls interposed between the female thread groove and themale thread groove, a housing for holding the ball nut rotatably, and afastening element for fixing the nut to the housing, wherein theelectric power steering apparatus is provided with a fall-out preventingmechanism for preventing the fastening element from falling out offwithin the housing.

According to the second invention, the fastening element does not comeoff the housing, and hence, even if the fastening element comes off andslackens, the ball nut does not move in excess of the predeterminedrange.

Further, in the electric power steering apparatus according to thesecond invention, the housing may be constructed of a first housingaccommodating the ball nut and the fastening element and a secondhousing fixed to the first housing, and the fall-out preventingmechanism may be a fastening element contact member, formed on thesecond housing, for regulating the fastening element from moving in thefall-out direction. With this contrivance, the fall-out preventingmechanism is formed simply by fastening integrally the first housing andthe second housing to each other.

Moreover, in the electric power steering apparatus according to thesecond invention, the fastening element may be a stopper ring fitted tothe first housing, and an interval between the stopper ring and thefastening element contact member may be, in an assembled state, setsmaller than a thickness of the stopper ring. This scheme prevents thestopper ring, even if on the verge of coming off due to a carelessoperation etc when assembled, from completely falling out because ofcoming into contact with the fastening element contact member.

In the electric power steering apparatus according to the secondinvention, the fastening element may be a ring bolt helically fitted tothe first housing, and an interval between the ring bolt and thefastening element contact member may be, in an assembled state, setsmaller than an effective helical-fitting length of the ring bolt to thefirst housing. With this contrivance, the ring bolt, even whenslackening due to the vibrations etc during driving, comes into thecontact with the fastening element contact member and is therebyprevented from completely falling out.

It is an object of the third invention of the present application toprovide a rack assist type electric power steering apparatus capable ofreducing a friction loss when operating and the number of components.

To accomplish the above object, according to the third invention, a rackassist type electric power steering apparatus comprises a ball nutrotationally driven by an electric motor, a ball screw mechanisminterposed between the ball nut and a rack shaft, and a bearing forrotatably supporting the ball nut, wherein the ball screw mechanism andthe bearing share a lubricant with each other.

Further, in the rack assist type electric power steering apparatusaccording to the third invention, it is preferable that a powertransmission mechanism be interposed between the electric motor and theball nut, and the power transmission mechanism, the ball screw mechanismand the bearing share the lubricant with each other.

In the rack assist type electric power steering apparatus according tothe third invention, the power transmission mechanism may be a gear typepower transmission mechanism or a chain type power transmissionmechanism or a belt type power transmission mechanism or a frictionroller type power transmission mechanism.

According to the third invention, there is no necessity for the shaftseal, whereby the friction loss caused by the rotations of the ball nutdecreases, and the enhanced feeling of the steering is attained.Besides, the decreases in the number of working processes for thesteering gear case etc and in the number of components, lead to theconsiderable reduction in the costs for the product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an in-cabin portion of an electricpower steering apparatus according to the first and second inventions;

FIG. 2 is a vertical sectional view showing a steering gear in the firstembodiment of the first invention;

FIG. 3 is an enlarged sectional view of a portion A in FIG. 2;

FIG. 4 is an enlarged sectional view showing the principal components ofthe steering gear in a second embodiment of the first invention;

FIG. 5 is an enlarged sectional view showing the principal components ofthe steering gear in a third embodiment of the first invention;

FIG. 6 is a front view showing the steering gear in a first embodimentof the second invention;

FIG. 7 is an enlarged sectional view showing a portion A in FIG. 6;

FIG. 8 is an explanatory view showing an operation in the firstembodiment of the second invention;

FIG. 9 is a sectional view showing the principal components of thesteering gear in a second embodiment of the second invention;

FIG. 10 is a perspective view showing an outline of construction of asteering apparatus in an embodiment of the third invention;

FIG. 11 is a vertical front view showing the principal components of thesteering gear in this embodiment; and

FIG. 12 is an enlarged view showing a portion A in FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several embodiments of the first, second and third inventions willhereinafter be described with reference to the accompanying drawings.

FIG. 1 is a perspective view showing an in-cabin portion of an electricpower steering apparatus in a first embodiment of the first invention. Amember indicated by the numeral 1 in FIG. 1 is a steering column. Thesteering column 1 supports an upper steering shaft 3 rotatably. Asteering wheel 5 is attached to an upper side end of the upper steeringshaft 3. A lower steering shaft 9 is joined via a universal joint 7 to alower side end of the shaft 3. Further, a steering gear mechanism 11constructed of a rack-and-pinion mechanism, a power assist mechanism etcis connected to a lower side end of the lower steering shaft 9.Referring again to FIG. 1, the numeral 13 represents a column cover forcovering the steering column 1, and the numeral 15 denotes tie rodsconnected to right and left ends of the steering gear mechanism 11.

FIG. 2 is a vertical sectional view showing principal components of thesteering gear mechanism 11 in the first embodiment of the firstinvention. FIG. 3 is an enlarged view showing a portion A in FIG. 2. Amember indicated by the numeral 21 in FIG. 2 is a rack-and-pinionhousing configuring a steering gear case. The rack-and-pinion housing 21accommodates a rack shaft 23 and a pinion (not shown) held therein,which constitute the rack-and-pinion mechanism. The rack shaft 23 has arack 25 provided on the left side in FIG. 2 and meshing with the pinion.Spherical joints 27, 27 supporting the tie rods 15, 15 are fixed to theright and left ends of the rack shaft 23.

The power assist mechanism has its shell configured by a gear housing 31formed at the right end, as viewed in FIG. 2, of the rack-and-pinionhousing 21, and by a ball screw housing 33 constituting the steeringgear case in cooperation with the rack-and-pinion housing 21 fastened bybolts to the gear housing 31. The gear housing 31, to which lowerportion the front end of an electric motor 35 is fastened by bolts,accommodates a drive gear (a spur gear in this embodiment) 37 fixed to amotor shaft (unillustrated), and a driven gear (a spur gear in thisembodiment) 39 meshing with the drive gear 37. Further, in the ballscrew housing 33 a ball nut 41 whose outer periphery is spline-fitted inthe driven gear 39, is rotatably held through a double-row angular ballbearing 43.

The rack shaft 23 is formed with a male thread groove 51, while the ballnut 41 is formed with a female thread groove 53, and a number of steelballs 55 defined as circulation balls are interposed between the malethread groove 51 and the female thread groove 53. Moreover, the ball nut41 is fitted with circulation pieces (not shown) for assisting thecirculation of the steel balls 55.

According to the first embodiment of the first invention, as illustratedin FIG. 3, an elastic bush 61 composed of an elastic member such as asynthetic rubber etc is interposed between the ball nut 41 and thebearing 43. The elastic bush 61 is formed with a cylindrical portion 63on which an inner race 43 a of the bearing 43 is fitted, and with flangemembers 65, 65 at both ends of the cylindrical portion 63. End surfacesof the inner race 43 a are brought into contact with inner end surfacesof the two flange members 65, 65. An outer end surface of one flangemember 65 comes into contact with a stepped portion 71 of the ball nut41, while an outer end surface of the other flange member 65 is securedby a stopper ring 73.

An operation in the first embodiment of the first invention willhereinafter be described.

When a driver rotates the steering wheel 5, a rotational force thereofis transferred to the steering gear mechanism 11 via the upper steeringshaft 3 and the lower steering shaft 9. The steering gear mechanism 11has a built-in rack-and-pinion mechanism for converting the rotationalinput into a rectilinear motion. With this mechanism, the rack shaft 23moves in any one of the right and left directions, and a steering angleof the wheels changes through the right and left tie rods 15, thusperforming a steering operation. On this occasion, within the powerassist mechanism, the electric motor 35, based on an output of anunillustrated steering torque sensor, rotates with a predeterminedrotational torque in a forward or reversed direction, and the rotationsthereof are decelerated and thus transferred to the ball nut 41 via thedriven gear 39. Upon rotations of the ball nut 41, a thrust force actson the male thread groove 51 of the rack shaft 23 through the steelballs 55 fitted in the female thread groove 53, thereby attaining asteering assist force.

According to the first embodiment of the first invention, the ball nut41 is held by the bearing 43 through the elastic bush 61, and therefore,even if somewhat a misalignment occurs between the ball nut 41 and therack shaft 23 due to lacks of working accuracy and assembling accuracywith respect to the parts, this misalignment is absorbed by adisplacement of the ball nut 41, thereby restraining a loss of drivingforce and abnormal abrasions of the respective portions. Further, theball nut 41 is so set as to be biased toward the electric motor 35 bythe elastic bush 61. With this setting, even if a backlash occursbetween the drive gear 37 and the driven gear 39, this backlash isabsorbed, and no noises are emitted.

FIG. 4 is an enlarged sectional view showing principal portions of thesteering gear in a second embodiment of the first invention. FIG. 5 isan enlarged sectional view showing principal portions of the steeringgear in a third embodiment of the first invention. Each of the secondand third embodiments takes substantially the same whole structure as inthe first embodiment discussed above, and their operations and effectsare the same as those in the first embodiment, however, differencestherebetween are a configuration of the elastic bush and a positionwhere this bush is disposed. To be specific, according to the secondembodiment, a couple of elastic bushes 61, 61 are interposedrespectively between the ball nut 41 and both ends of the inner race 43a of the bearing 43, thus scheming to improve an assembly workabilityetc. Further, according to the third embodiment shown in FIG. 5, thecouple of elastic bushes 61, 61 are interposed between a ball screwhousing 33 and an outer race 43 b of the bearing 43, whereby the ballnut 41 and the bearing 43 make integral displacements with respect tothe ball screw housing 33.

The explanations of the specific embodiments of the first invention,which have been discussed so far, come to an end, however, the mode ofthe first invention is not limited to the embodiments discussed above.For instance, the electric motor and the ball nut are connected by useof the couple of spur gears and may also be connected by three or moregears including an idle gear or by a helical gear train. Further, themotor shaft of the electric motor and the central axis of the ball nutare not necessarily disposed in parallel, and it follows that theybecome orthogonal in the case of combining a worm wheel with a pinionand using a bevel gear etc. Moreover, the configurations, from the shapeof the elastic member and the whole construction of the steering geardown to the structure of the power assist mechanism, are not confined tothe exemplifications in the embodiments discussed above, and can be, ifwithin the range that does not depart from the concept of the presentfirst invention, properly changed based on requests in terms of designsand specifications.

As discussed above, the electric power steering apparatus according tothe first invention includes the ball nut rotationally driven by theelectric motor through the gear train and having a female thread grooveformed in the inner periphery thereof, the rack shaft disposed throughthe ball nut on the axis thereof and having a male thread groove soformed in its outer periphery as to face to the female thread groove,the ball screw mechanism having the plurality of circulation ballsinterposed between the female thread groove and the male thread groove,and the housing for holding the ball nut rotatably through the bearing.In the thus constructed electric power steering apparatus, the elasticmembers are interposed between the housing and the ball nut in order topermit the ball nut to make the predetermined displacement. Therefore,even if the misalignment occurs between the ball nut and the rack shaft,the ball nut displaces owing to deformations of the elastic members,thereby canceling this misalignment. In addition, the backlash causedbetween the gears due to a defect in working of the housing etc is alsoobviated by biasing the ball nut in the direction right-angled to theaxial direction with the elastic members.

An outline of a configuration of the electric power steering apparatusin a first embodiment of the second invention is illustrated in FIG. 1as in the first invention.

FIG. 6 is a front view showing a steering gear mechanism 11 in the firstembodiment of the second invention. A member indicated by the numeral 21in FIG. 6 is a rack-and-pinion housing configuring a steering gear case.The rack-and-pinion housing 21 accommodates a rack shaft 23 and a pinion(not shown) held therein, which constitute the rack-and-pinionmechanism. The rack shaft 23 has a rack 25 provided on the left in FIG.6 and meshing with the pinion (not shown). Spherical joints 27, 27supporting the tie rods 15, 15 are fixed to the right and left ends ofthe rack shaft 23.

The power assist mechanism has its shell configured by a gear housing131 defined as a second housing formed at the left end of therack-and-pinion housing 21, and by a ball screw housing 133 defined as afirst housing fastened by bolts to the gear housing 131 and constitutingthe steering gear case in cooperation with the rack-and-pinion housing21. The front end of the electric motor 35 is fastened by bolts to alower portion of the gear housing 131.

A drive gear 137 fixed to a motor shaft (unillustrated) of the electricmotor 35 and a driven gear 139 meshing with the drive gear 137 throughan idle gear 138, are housed in between the gear housing 131 and theball screw housing 133. Further, a ball nut 141 provided with the drivengear 139 at its end is rotatably held in the ball screw housing 133. Theball nut 141 is constructed of a ball nut body 143 and a double-rowangular ball bearing 145 fitted on the ball nut body 143.

The double-row angular ball bearing 145 is structured such that steelballs 157 are interposed in two rows between a couple of inner races151, 153 and an outer race 155, wherein the inner races 151, 153 aresecured by a stopper ring 159 fitted on the ball nut body 143. With thisstructure, the ball nut body 143 is integrated with the double-rowangular ball bearing 145.

The rack shaft 23 is formed with a male thread groove 161, while theball nut body 143 is formed with a female thread groove 163. A number ofsteel balls 165 serving as the circulation balls are interposed betweenthe male thread groove 161 and the female screw groove 163. Moreover,the ball nut body 143 is fitted with circulation pieces (not shown) forassisting the circulation of the steel balls 165.

According to the first embodiment of the second invention, the ballscrew housing 133 has an annular groove 171 formed in along its innerperipheral surface on the side of the gear housing 131. An outerperipheral portion of a C-shaped stopper ring 173 fixedly engaging withthe outer race 155 of the double-row angular ball bearing 145 is fittedin this annular groove 171. On the other hand, the gear housing 131 isprovided with a fastening element contact member 175 protruding in aface-to-face relationship with the C-shaped stopper ring 173. Then, aninterval t (between the fastening element contact member 175 and theleft end surface, as viewed in the Figure, of the annular groove 171)between the fastening element contact member 175 and the C-shapedstopper ring 173, is set significantly smaller than a thickness B of theC-shaped stopper ring 173.

An operation of the first embodiment of the second invention willhereinafter be described.

On the occasion of assembling the steering gear mechanism 11, anassembler fits the ball nut 141 into the ball screw housing 133, andthereafter prevents a shift of the ball nut 141 by fitting the C-shapedstopper ring 173 into the annular groove 171.

According to the first embodiment of the second invention, the clearancet between the fastening element contact member 175 and the left endsurface, as viewed in the Figure, of the annular groove 171 is setsmaller then the width B of the C-shaped stopper ring 173, therebypreventing the C-shaped stopper ring 173 from falling out of the ballscrew housing 133.

With this contrivance, according to the first embodiment of the secondinvention, even if unexpected load is applied to the nut and C-shapedstopper ring 173 comes off from the groove of housing, the ball bearingcould not move in axial direction, and the steer function is secured.

FIG. 9 is a vertical sectional view showing principal portions of thesteering gear mechanism 11 in a second embodiment of the secondinvention. The second embodiment of the second invention also adopts thesame whole construction as in the first embodiment of the secondinvention, however, the ball nut 141 (the double-row angular ballbearing 145) is secured by a ring bolt 181. Then, an interval t′ betweenthe fastening element contact member 175 provided on the gear housing 31and the ring bolt 181, is set significantly smaller than an effectivehelical fitting length 1 between the ring bolt 181 and the ball screwhousing 133.

In the case of this second embodiment, the ring bolt 181, even whenslackening due to vibrations etc during driving, does not fall outbecause of its end surface being contact-engaged with the fasteningelement contact member 175. Consequently, as in the first embodiment ofthe second invention, the quantity of axial shift of the ball nut 141 isregulated at the maximum t′, with the result that the unagreeablefeeling of the steering etc does not occur.

The specific embodiments, which have been discussed so far, comes to anend of their explanations, however, the mode of the second invention isnot limited to the first and second embodiments described above. Forinstance, according to the embodiments discussed above, the rotations ofthe electric motor are transmitted via the gear to the ball nut and mayalso be transmitted thereto via a belt. Further, in the embodimentsdiscussed above, the component of the ball nut involves the use of thedouble-row angular ball bearing, however, a double-row tapered rollerbearing, a slide bearing etc may also be used. Moreover, the fasteningelement of the ball nut may involve the use of elements excluding theC-shaped stopper ring and the ring bolt, and the mechanism forpreventing the fall-out of the fastening element may involve the use ofmembers other than the fastening element contact member provided on thegear housing. Furthermore, the whole configuration of the steering gearand the structure of the power assist mechanism are not limited to theexemplifications in the embodiments discussed above, and can be, ifwithin the range that does not deviate from the concept of the secondinvention, properly changed based on requests in terms of designs andspecifications.

As discussed above, the electric power steering apparatus according tothe second invention includes the ball nut rotationally driven by theelectric motor and having a female thread groove formed in the innerperiphery of the ball nut, the rack shaft disposed through the ball nuton the axis thereof and having a male thread groove so formed in itsouter periphery as to face to the female thread groove, the plurality ofcirculation balls interposed between the female thread groove and themale thread groove, the housing for holding the ball nut rotatably, andthe fastening element for fixing the nut to the housing. The thusconstructed electric power steering apparatus is provided with thefall-out preventing mechanism for preventing the fastening element fromfalling out within the housing, and hence, even if the fastening elementcomes off and slackens, the ball nut does not move in excess of thepredetermined range, whereby it is possible to actualize preventing theoccurrence of the unagreeable feeling of the steering and so on.

FIG. 10 is a perspective view showing an outline of configuration of arack assist type electric power steering apparatus in an embodiment ofthe third invention. A member indicated by the numeral 1 in FIG. 10 is asteering column. The steering column 1 supports an upper steering shaft3 rotatably. A steering wheel 5 is attached to an upper side end of theupper steering shaft 3. A lower steering shaft 9 is joined via auniversal joint 7 to a lower side end of the shaft 3. A steering gearmechanism 11 constructed of a rack-and-pinion mechanism, a power assistmechanism etc is connected to a lower side end of the lower steeringshaft 9. Referring again to FIG. 10, the numerals 15, 15 represent tierods connected to right and left ends of the steering gear mechanism 11.

FIG. 11 is a vertical plan view showing principal components of thesteering gear mechanism 11 in this embodiment. FIG. 12 is an enlargedview showing a portion A in FIG. 11. A member indicated by the numeral21 in these Figures is a rack-and-pinion housing configuring a steeringgear case. The rack-and-pinion housing 21 accommodates a rack shaft 23and a pinion (not shown) held therein, which constitute therack-and-pinion mechanism. The rack shaft 23 has a rack 25 provided onthe left as viewed in the Figures and meshing with the pinion (notshown). Spherical joints 27, 27 supporting the tie rods 15, 15 are fixedto the right and left ends of the rack shaft 23.

The power assist mechanism has its shell configured by a gear housing231 defined as a second housing formed at the right end, as viewed inFIG. 11, of the rack-and-pinion housing 21, and by a ball screw housing233 defined as a first housing fastened by bolts to the gear housing 231and constituting the steering gear case in cooperation with therack-and-pinion housing 21. As shown in FIG. 10, an electric motor 35 isfastened by bolts to the front portion of the gear housing 231. Notethat the electric motor 35 is depicted downward for the explanatoryconvenience in FIG. 11.

A drive gear 237 fixed to a motor shaft (unillustrated) of the electricmotor 35 and a driven gear 239 meshing with the drive gear 237 throughan idle gear 238, are provided between the gear housing 231 and the ballscrew housing 233. These gears 237, 238, 239 constitute a gear train 240serving as a gear type power transmission mechanism.

A ball nut 241 having its end portion provided with the driven gear 239is rotatably held in the ball screw housing 233. The ball nut 241 isconstructed of a ball nut body 243 and a double-row angular ball bearing245 fitted on the ball nut body 243.

The rack shaft 23 is formed with a male thread groove 251, while theball nut body 243 is formed with a female thread groove 253, and amultiplicity of steel balls 525 defined as circulation balls areinterposed between the male thread groove 521 and the female threadgroove 253. A ball screw mechanism 257 is constructed of thesecomponents. Moreover, the ball nut body 243 is fitted with circulationpieces (not shown) for assisting the circulation of the steel balls 255.

As illustrated in FIG. 12, an air space encompassed by the gear housing231 and the ball screw housing 233 is filled with lithium soap grease(which will hereinafter simply be called a grease) 261 as a lubricant.In this embodiment of the third invention, the grease 261 fills or isexchanged when assembling the steering gear 11 or disassembling thisgear 11 for maintenance, and serves for lubrications between the geartrain 240, the ball screw mechanism 257 and the double-row angular ballbearing 245.

This embodiment of the third invention does not use the shaft seal forpreventing a mixture of the grease in the conventional apparatus, andhence a friction loss caused by the rotations of the ball nut 241decreases, thereby enhancing the feeling of the steering. Further,neither the gear housing 231, nor the ball screw housing 233 nor theball nut 241 needs working a seal seat or seal slide surface for theshaft seal, whereby the costs for the product remarkably decreases alongwith no use of the shaft seal.

The specific embodiment of the third invention, which has been discussedso far, comes to an end of its explanation, however, the mode of thethird invention is not limited to the embodiment described above. Forexample, in the embodiment discussed above, what has been exemplified asthe power transmission mechanism is the gear type power transmissionmechanism including the single idle gear, however, a chain type powertransmission mechanism or a belt type power transmission mechanism or afriction roller type power transmission mechanism may also be adopted,and there can be also applied a so-called coaxial type powertransmission mechanism in which the ball nut is integrated with therotor of the electric motor. Further, the double-row angular ballbearing has been exemplified as the bearing, however, other types ofbearing may also be used. Further, solid-state lubricants and greasesother than lithium soap grease are, if capable of meeting the conditionssuch as a lubricating performance, a durability etc, usable as thelubricant. For others, the whole configuration of the steering gear, thestructure of the power assist mechanism and so on can be, if within therange that does not deviate from the concept of the third invention,properly changed based on requests in terms of designs andspecifications.

As discussed above, according to the third invention, the rack assisttype electric power steering apparatus includes the ball nutrotationally driven by the electric motor, the ball screw mechanisminterposed between the ball nut and the rack shaft, and the bearing forrotatably supporting the ball nut. In the thus constructed electricpower steering apparatus, the ball screw mechanism shares the lubricantwith the bearing, which eliminates the necessity for the shaft seal, andso on. Owing to the above-mentioned, the friction loss caused by therotations of the ball nut decreases, and the enhanced feeling of thesteering is attained. Besides, the decreases in the number of workingprocesses for the steering gear case etc and in the number ofcomponents, lead to the considerable reduction in the costs for theproduct.

What is claimed is:
 1. A rack assist type electric power steeringapparatus comprising: a ball nut rotationally driven by an electricmotor; a ball screw mechanism interposed between said ball nut and arack shaft; and a bearing for rotatably supporting said ball nut,wherein said ball screw mechanism and said bearing share a lubricantwith each other.
 2. A rack assist type electric power steering apparatusaccording to claim 1, wherein power transmission means is interposedbetween said electric motor and said ball nut, and said powertransmission means, said ball screw mechanism and said bearing share thelubricant with each other.
 3. A rack assist type electric power steeringapparatus according to claim 2, wherein said power transmission means isa gear type power transmission mechanism or a chain type powertransmission mechanism or a belt type power transmission mechanism or afriction roller type power transmission mechanism.